1. Field of the Invention
The present invention relates to an alignment marks and a method of forming the same and, more specifically, to alignment marks for registration of masks of respective layers when a number of layers are successively formed on a substrate, and to a method of forming the same.
2. Description of the Background Art
Alignment marks for registration of masks of stacked layers used in forming elements on a semiconductor wafer have been known. FIG. 6 is a schematic diagram showing a full structure of a conventional semiconductor wafer having alignment marks formed thereon. Referring to FIG. 6, a plurality of LSI chips 101 are formed on a semiconductor wafer 100. Regions 102a and 102b for forming alignment marks are provided in the directions of X and Y axes along the LSI chips 101. In the region 102a, an alignment mark for registration in the Y axis direction is formed, while in the region 102b, an alignment mark for registration in the X axis direction is formed.
FIG. 7A is an enlarged plan view of the region 102b for forming the alignment mark, FIG. 7B is a front section taken along the line A--A of the alignment mark forming region shown in FIG. 7A, and FIG. 7C is a waveform showing a signal waveform provided by light diffracted from the conventional alignment mark shown in FIGS. 7A and 7B. Referring to FIG. 7A, alignment marks 1c are formed spaced apart from each other by a prescribed distance, in the longitudinal direction, and a plurality of (5 to 10) columns of marks are arranged. The cross section of the alignment mark forming region 102b of FIG. 7A taken along the line A--A is as shown in FIG. 7B. More specifically, alignment marks 1c of convexes and concaves are formed spaced apart by a prescribed distance on the substrate 1. When He-Ne laser beam 200 having longitudinal range of irradiation is emitted in the direction of the arrow as shown in FIG. 7A, a signal waveform such as shown in FIG. 7C of the light diffracted from the alignment marks 1c of FIG. 7B is provided. Registration of the masks is done based on the signal waveform.
FIGS. 8A to 8C are cross sections showing the steps of manufacturing the conventional alignment marks shown in FIG. 7A and 7B. Referring to FIG. 8A to 8C, the method of manufacturing the conventional alignment marks will be described. First, as shown in FIG. 8A, a resist pattern 21 is formed on a region on a substrate 1 where the alignment marks 1c, which will be described later, are to be formed. By effecting etching using the resist patten 21 as a mask, alignment marks 1c having prescribed height are formed. Then, as shown in FIG. 8C, the resist pattern 21 is removed. The conventional alignment marks 1c are formed in this manner.
FIG. 9A shows ideal forms of an upper layer film and a resist film formed on the alignment marks shown in FIG. 7B. Referring to FIG. 9A, an ideal upper layer film 22 is formed conformably on the substrate 1, and an ideal resist film whose surface height made even is formed on the upper layer film 22. FIG. 9B shows a signal waveform provided by the light diffracted from the alignment marks shown in FIG. 9A. Referring to FIG. 9B, the signal waveform provided by the structure shown in FIG. 9A is similar to that of FIG. 7C. In the actual process of manufacturing, the upper layer film 22 is patterned based on the signal waveform provided with the alignment marks 1C formed on the substrate 1 as a reference, as shown in FIG. 9A. Namely, based on the signal waveform shown in FIG. 9B, the resist film 23 formed on the upper layer film 22 is patterned at first, and the upper layer film 22 is etched by using the patterned resist film 23. Thus, the conventional step of mask registration for forming elements such as LSI chips are done using the concave and convex shaped alignment marks 1c. When elements having a plurality of layers stacked one after another are to be formed, concave and convex shaped alignment marks are formed on respective layers as needed, and mask registration of the layer formed there on is done based on such marks.
As described above, conventionally, registration of masks during manufacturing elements is done by forming concave and convex shaped alignment marks 1c and by using the signal waveforms of the light diffracted from the alignment marks 1c.
However, accurate signal waveforms may not be provided dependent on the surface condition (roughness) of the upper layer film formed on the alignment marks 1c or on the shapes of the resist films formed further thereon. FIG. 10A is a cross section showing the shapes of alignment marks formed on a substrate whose surface is rough, and FIG. 10B shows a signal waveform provided by the light diffracted from the alignment marks shown in FIG. 10A. Referring to FIG. 10A, when a metal film such as Al or W is formed by spattering, the surface of the metal film, which is the substrate 11 on which the alignment marks are to be formed, becomes rough dependent on the conditions of film formation by thermal processing. When the alignment marks 1c are formed in this condition, the surface of the alignment marks 1c becomes rough. Even if a normal insulating film 32 and an ideal resist film 33 having the surface height even are formed on the alignment marks 1c, the signal waveform of the diffracted light provided by the shapes of FIG. 10A will be as shown in FIG. 10B, since minute rough shapes on the surface of the substrate 11 increases diffuse reflection. And it is difficult to accurately registrate masks based on such a waveform as shown in FIG. 10B.
FIG. 11A shows a cross section in which a normal insulating film and an actual resist film are formed on alignment marks the surfaces of which are smooth and flat, and FIG. 11B shows a signal waveform provided by the light diffracted from the alignment marks shown in FIG. 11A. Referring to FIG. 11A, when a normal insulating film 42 or the like is formed without thermal processing after deposition or formed by thermal oxidation on the substrate 1, the surface of the insulating film 42 has almost ideal shape as that of the ideal upper layer film 22 shown in FIG. 9A. However, as to the resist film 43 formed on the normal insulating film 42, it is difficult to provide a symmetrical coverage such as shown in FIG. 9A. Namely, in the actual step of resist application, provision of symmetrical coverage is difficult due to distortion and deviation caused by the concave and convex shapes of the alignment marks 1c, resulting in asymmetrical resist coverage. When such asymmetrical resist 43 is formed, the signal waveform becomes as shown in FIG. 11B, and the peak of the diffraction light is off the center of the mark shown in FIG. 9B. If registration of masks is done by using such a signal waveform of the diffraction light, accurate registration is difficult.
More specifically, in the prior art, when alignment marks 1c are formed on a surface of a metal film which is made rough during spattering or thermal processing, peaks of the waveform of the diffraction light do not appear, making it difficult to accurately carry out registration of masks. Even if normal insulating film having flat surface is formed on alignment marks 1c with flat surfaces, the resist film 43 formed on the normal insulating film 42 has asymmetrical coverage. Therefore, the peaks of the signal waveform of the diffraction light are off the center, and in this case also, accurate registration is difficult.
Namely, in the prior art, it is difficult to effectively prevent degradation of registration accuracy caused by asymmetrical resist coverage, and it is difficult to form alignment marks providing accurate mask registration signals when the substrate surface is rough.